The release of neurotransmitters, neuromodulators and hormones from synapses and neurosecretory cells involves exocytosis: the fusion of the storage granule membrane (synaptic vesicle, chromaffin granule, neurohypophoseal granule, etc.) with the cell plasma membrane. We have chosen to examine the divalent metal ion-promoted fusion of both artificial and biological membranes as models for the reactions involved in the cellular processes. The kinetics of membrane fusion reactions in a model system consisting of sonicated phospholipid vesicles are being studied using a computer-controlled multisignal stopped-flow rapid mixing apparatus. Membranes labeled with fluorescent phospholipids are rapidly mixed with calcium and/or other agents which promote membrane fusion and the reactions are followed by the relaxation of several optical signals. The machine can presently acquire data from three different photomultipliers. A new version of the machine will be able to acquire four channels of data from material excited by two different wavelengths of light. Initial results demonstrate that the rates of fusion of both small unilamellar sonicated phospholipid vesicles (SUV's) and larger unilamellar vesicles formed by reverse phase evaporation (REV's) are limited by the aggregation of the vesicles. Fusion of REV's results in little to no loss of materials stored in the vesicle lumens while the rapid loss of materials from the SUV's suggest that they are poor models for the biological processess. Methods for placing the fluorescent phospholipid probes into biological membranes have been developed, which will allow the study of the fusion processes of these particles.